What are the files in the /tmp folder and can I delete them?
To expand on cegatons answer - /tmp/ is the default location for some data paths unless you change them.

The main one is the render output, unless you change the output filepath before rendering, your renders will be saved to tmp. Changing the output path before saving your startup file will prevent any renders going into tmp.

Simulation cache files can also be saved in tmp. While most prevent saving of external cache data until the blend file is saved, the fluid simulation will save it's cache files to tmp if you haven't saved the blend file first. If you then save the file, the path will still point to data in tmp instead of moving the data to where you saved your blend.

The ocean modifier is another that will store it's data in tmp if you haven't saved the file before baking the simulation.

If you save your blend file to disk before starting any simulations then the data will first get saved within the same folder as your blend file.